1. Field of the Invention
The present invention relates to field-effect transistors using a nitride semiconductor.
2. Description of Related Art
In recent years, field-effect transistors typified by a high electron mobility transistor (HEMT) using a GaN-based compound semiconductor (hereinafter referred to as a “GaN-based FET”), instead of a GaAs-based compound semiconductor, have attracted attention as a next-generation high-speed FET. The GaN-based compound semiconductor has a wide bandgap, and a high saturated electron mobility as estimated from its electron effective mass. The GaN-based compound semiconductor could achieve high frequency devices operable at high temperature with a higher output power and a high breakdown voltage. For this reason, the GaN-based compound semiconductors have been studied.
The GaN-based FET using the GaN-based compound semiconductor includes, for example, a buffer layer, a GaN layer, and a AlGaN layer stacked on an insulating sapphire substrate in that order. The GaN-based FET also includes a source electrode, a gate electrode, and a drain electrode which are formed over the upper surface of the AlGaN layer. The HEMT with the above structure is operated by a high concentration of carrier generated from both effects of spontaneous polarization and piezoelectric polarization, unlike a GaAs compound semiconductor FET driven by a carrier generated by doping impurities. Specifically, when the AlGaN layer is grown on the GaN layer, both effects of the spontaneous polarization and piezoelectric polarization generate positive fixed charges on the hetero-interface, and negative polarized charges on the surface of the AlGaN layer. The concentration of polarized charges changes depending on the composition or thickness of the AlGaN layer. The AlGaN/GaN heterostructure causes a very large sheet electron concentration of about 1×1013/cm2. The application of an electric field to between ohmic electrodes formed in the hetero structure allows a current to flow due to transport of charges in a high electron concentration of about 1×1013/cm2.
The GaN-based FET can fabricate devices with a high breakdown voltage and a low on resistance as compared to the conventional Si power device (MOSFET or IGBT). The use of the GaN-based FET can be expected to reduce the power consumption of the device.
However, the field-effect transistor made of a nitride semiconductor disadvantageously makes it difficult to perform a normally-off operation.
In view of the foregoing problem, JP 2006-339561 A has an object of providing a normally-off type field-effect transistor using a nitride semiconductor. Specifically, a gate electrode is formed over a laminated structure made of the nitride semiconductor via a high concentration p-type GaN layer, so that the gate electrode is ohmic-connected to the high concentration p-type GaN layer to thereby form a p-n junction in a gate region as disclosed in JP 2006-339561 A.
JP 2008-153330 A discloses a technique for obtaining stable enhancement-mode GaN-HEMT characteristics. The technique involves forming an i-GaN selecting regrown layer 29 of about 1 nm in thickness over a p-GaN selecting regrown layer 28 formed for the same purpose as that of JP 2006-339561 A to thereby prevent surface oxidation of the p-GaN selecting regrown layer 28.
However, the field-effect transistors using the nitride semiconductor as disclosed in JP 2006-339561 A and JP 2008-153330 A do not have a sufficiently high threshold voltage even though the normally-off operation can be achieved.
Further, the field-effect transistor disclosed in JP 2006-339561 A and JP 2008-153330 A disadvantageously has a large gate leak current in driving (when turned on), which leads to a large loss of performance. In order to suppress the gate leak current, such a field-effect transistor needs to be driven with a small gate bias. A depletion layer located in a position where a channel is to be formed cannot be sufficiently eliminated by the gate bias with a part of the depletion layer remaining in the channel.
Thus, a resistance between a source and a drain of the transistor cannot be sufficiently suppressed, thereby reducing the maximum current obtained, which results in a field-effect transistor having a large loss of the current with a large gate leak current.